Plasma Panel Based Radiation Detector

ABSTRACT

A radiation counting detector includes a first substrate and a second substrate that is generally parallel to first substrate and forms a gap with the first substrate. A gas is contained within the gap. A photocathode layer is coupled to one side of the first substrate and faces the second substrate. A first electrode is coupled to the second substrate and a second electrode is electrically coupled to the first electrode. A first impedance is coupled to the first electrode and a second impedance is coupled to the second electrode. A power supply is coupled to at least one of the electrodes. A first discharge event detector is coupled to the first impedance and a second discharge event detector is coupled to the second impedance. The radiation counting detector further includes a plurality of pixels, each capable of outputting a gas discharge pulse upon interaction with radiation received from the photocathode. Each gas discharge pulse is counted as having an approximately equal value. The radiation counting detector further includes circuitry for detecting if a gas discharge pulse is output from the pixels, and for counting each gas discharge pulse as an individual event.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/854,919, filed Oct. 28, 2006, the specification ofwhich is herein incorporated by reference, and U.S. Provisional PatentApplication No. 60/878,508, filed Jan. 4, 2007, the specification ofwhich is herein incorporated by reference.

FIELD OF THE INVENTION

Embodiments of the present invention are directed to radiationdetection. More particularly, one embodiment of the present invention isdirected to a plasma panel based method and apparatus for radiationdetection.

BACKGROUND INFORMATION

Many useful applications, such as the detection of radioactive material,computer-assisted tomography (“CAT”), digital radiology, opticaldetectors, etc., rely on the detection of ionizing radiation (e.g.,X-ray and gamma-ray photons and/or high energy particles—both neutraland charged) as well as “non-ionizing” photons. Non-ionizing photons,sometimes referred to as “optical” photons, are photons generallyfalling within the energy range from the ultraviolet (“UV”) tonear-infrared (“IR”), and are commonly detected by various types ofdevices such as complementary metal-oxide-semiconductor (“CMOS”),charge-coupled devices (“CCDs”), avalanche photodiodes (“APD”s),photomultiplier tubes (“PMT”s), etc. Generally, the low energy end ofthe X-ray region begins at about 10 nm, which also approximately definesthe high energy end of the “optical” photon region. However, differentdescribed energy regions broadly overlap and so descriptive terms suchas “ionizing” and “non-ionizing” and “optical” in reference to a type ofradiation are merely used to label a spectral region or particle energybut are not narrowly defined. For example, a given UV photon can be bothionizing and non-ionizing depending upon the interacting media. Evenphotons in the “visible” region can be ionizing with respect to certainmaterials.

Many prior art radiation detectors are proportional detectors. Ingeneral, proportional detectors store charge in capacitors or othermeans, and the total amount of stored charge is proportional to detectedradiation. Proportional detectors operate on the principle of linear gasmultiplication, and the final charge measured is proportional to thenumber of original ion pairs created within the gas by the incidentradiation, which is proportional to the energy of the incidentradiation. Proportional detectors typically require amplificationcircuitry in order to measure the charge.

Recently, new types of proportional gas-based radiation detector deviceshave been developed, including micropattern gas detectors such ascascaded Gas Electron Multipliers (“GEM”). These devices, which havebeen under development primarily for use in high-energy and nuclearphysics, have many desirable properties as proportional gas detectors,but are limited to gains on the order of about 10⁶. Their use however,has been held back in large part due to avalanche-induced secondaryeffects associated with ion, electron, photon and metastable speciesfeedback, as well as photocathode degradation caused by ion impact.

Based on the foregoing, there is a need for a radiation sensor with highresolution capability, fast pixel response, minimal dead-time, highgain, improved radioisotope identification, low power consumption, athin profile and physically rugged, that can be manufactured in largesizes relatively inexpensively.

SUMMARY OF THE INVENTION

One embodiment of the present invention is a radiation counting detectorthat includes a first substrate, and a second substrate that isgenerally parallel to first substrate and forms a gap with the firstsubstrate. A gas is contained within the gap. A photocathode layer iscoupled to one side of the first substrate and faces the secondsubstrate. A first electrode is coupled to the second substrate and asecond electrode is electrically coupled to the first electrode. A firstimpedance is coupled to the first electrode and a second impedance iscoupled to the second electrode. A power supply is coupled to at leastone of the electrodes. A first discharge event detector is coupled tothe first impedance and a second discharge event detector is coupled tothe second impedance. The radiation counting detector further includes aplurality of pixels, each capable of outputting a gas discharge pulseupon interaction with radiation received from the photocathode. Each gasdischarge pulse is counted as having an approximately equal value. Theradiation counting detector further includes circuitry for detecting ifa gas discharge pulse is output from the pixels, and for counting eachgas discharge pulse as an individual event.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a columnar-discharge plasma panelphotosensor (“PPPS”) detector that includes a scintillation plate inaccordance with one embodiment of the present invention.

FIG. 2 is a perspective view of a surface-discharge PPPS detector with aparallel/rectilinear electrode pattern in accordance with one embodimentof the present invention.

FIG. 3 is a perspective view of a barrier structure in accordance withone embodiment that can be used with the disclosed detectors.

FIG. 4 is a perspective view of a vertically-stacked PPPS scintillationdetector in accordance with one embodiment that includes stackedscintillation plates.

FIG. 5 is a perspective view and schematic showing a sequential multiplescattering event within a vertically-stacked PPPS-scintillation detectorin accordance with another embodiment of the present invention.

FIG. 6 is a cross-sectional view of a microcavity-PPPS detectorconfigured to operate as a DC device in accordance with one embodimentof the present invention.

FIG. 7 is a block diagram of a counting circuit in accordance with oneembodiment of the present invention.

FIG. 8 is a cross-sectional view of a PPPS-total internal reflectancefluorescence (“TIRF”) cell for collecting fluorescent photons inaccordance with one embodiment of the present invention.

FIG. 9 is a cross-sectional view of a PPPS-TIRF dual cell in accordancewith one embodiment of the present invention.

FIG. 10 is a perspective view of a surface-discharge, plasma panelsensor (“PPS”) based neutron detector in accordance with one embodimentof the present invention.

FIG. 11 is a perspective view of a surface-discharge PPPS detector witha circular electrode pattern in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION

One embodiment of the present invention is a device that enhances thecapability and/or reduces the cost for detecting radiation for ionizingparticles and photons, as well as non-ionizing photons such as “optical”photons. Embodiments do not record the magnitude of a given cell gasdischarge as do most prior art detectors that operate in the linearregion as proportional devices, but instead operates in the non-linearregion with extremely high gain and is able to employ Geiger-Muellertype counting methods in assigning essentially the same value to eachevent regardless of the cell discharge magnitude. Embodiments of thepresent invention are therefore inherently digital in nature and can behighly pixelated, can utilize low cost materials and fabricationprocesses, and are extremely rugged and capable of operating underchallenging ambient conditions including high magnetic fields. Oneembodiment comprises a plasma panel device designed to operate as a highgain, highly pixelated, digital photodetector by incorporating, in part,an internal photocathode that can be optically coupled to ascintillation plate and/or various other luminescent materials.

One embodiment couples a plasma panel sensor (“PPS”) disclosed in U.S.patent application Ser. No. 11/155,660, the disclosure of which isherein incorporated by reference, with an internal photocathodesensitive to “optical” photons (and eliminates the PPS conversionlayer), to form a hybrid gaseous-solid state device that is referred toas a plasma panel photosensor (“PPPS”) to achieve unexpected results. Inall embodiments, the resulting PPPS is a highly-pixelated andinherently-digital radiation detector, and requires little or no signalamplification and can be configured to achieve high angular resolution(e.g., Compton telescope) with good spectroscopic sensitivity across anextremely broad energy range—e.g. from low-keV to high-MeV for ionizingradiation, and from the UV to near-IR for “optical” photons.

The PPS disclosed in U.S. patent application Ser. No. 11/155,660includes a conversion plate or conversion layer that absorbs “ionizing”radiation, but does not include photocathodic type materials andtherefore is able to be directly exposed to the outside atmosphere andambient illumination without adverse consequence. In contrast,embodiments of the present invention do not generally employ theconversion plate or conversion layer as described in U.S. patentapplication Ser. No. 11/155,660, but include an internal photocathode orphotocathode layer adapted to detect “optical” (e.g. UV-visible) photonsand so need to be generally protected from exposure to the outsideatmosphere and ambient illumination. Embodiments of the presentinvention therefore generally interact with “ionizing” radiationindirectly by being coupled, for example, to a scintillation plate.

Embodiments of the PPPS detector offer a number of significantimprovements relative to known radiation detectors, including: (1) itcan be used for high-resolution digital radiography, radiation sourceimaging, computed tomography, source isotope identification, neutronactivation spectroscopy, free-space optical communication, chemical andbiological species detection, optical photon based chemical sensorsand/or biosensors; (2) it is a highly-pixelated, high-gain, intensitysensitive, digital detector without the use of A/D converters; (3) it isa rugged, large area, flat-panel type detector with excellentposition-sensitive capability; (4) it provides high-level performanceunder challenging and/or difficult environmental conditions withinsensitivity to magnetic fields, low susceptibility to radiation damageand generally low power consumption; (5) it has a potentiallyorder-of-magnitude cost reduction compared to a number of currentdetection techniques, especially for large area, rapid response,radiation detection systems; and (6) it has a thin devicecross-sectional profile facilitating operation in a variety ofvertically-stacked array configurations and apparatus for enhancedefficiency and/or improved performance including system designs such ascoded-apertures, double-scatter kinematics, Compton telescopes, etc.

FIG. 1 is a perspective view of a columnar-discharge plasma panelphotosensor (“PPPS”) detector 10 that includes a scintillation plate 12in accordance with one embodiment of the present invention. PPPSdetector 10 includes a front substrate 16 optically coupled through anoptical coupling layer 14 to the scintillation plate 12. PPPS detector10 includes a back substrate 18 that is separated from front substrate16 by a gas-filled gap or discharge gap 20 that is filled with dischargegas 34. In one embodiment, scintillation plate 12 also functions as thefront substrate and therefore optical coupling layer 14 is not included.

Detector 10 further includes X-electrodes 22 (e.g., rows), andY-electrodes 24 (e.g., columns). Detector 10 also includes aphotocathode layer 26 and a back substrate conductive layer 28 that canbleed off accumulated charge on a back substrate electrode insulationlayer 32. Detector 10 further includes a front substrate “ultra-thin”protective dielectric layer 30 that can protect photocathode 26 fromdegradation due to ion bombardment and maintain the integrity ofX-electrodes 22, which along with the Y-electrodes 24 define the PPPSpixel structure.

In one embodiment, the charge that collects on the back dielectricsurface between adjacent Y-electrodes 24 does not interfere with theoperation of detector 10 and so conductive layer 28 and electrodeinsulation layer 32 are not needed. In this embodiment, Y-electrodes 24are deposited directly on the interior surface of back substrate 18 inan embodiment that substrate 18 is formed of a dielectric material suchas glass. In one embodiment, photocathode 26 can bleed off any storedcharge that might tend to accumulate on ultra-thin protective dielectriclayer 30. In another embodiment a transparent conductive coating (e.g.ITO or SnO) can be deposited on front substrate 16 directly beneathphotocathode 26 to improve the conductivity of the photocathode andassist in removal of stored charge from protective dielectric layer 30.

Detector 10 of FIG. 1 does not include barriers, and in one embodimentoperates in a direct current (“DC”) mode. In another embodiment,detector 10 may operate in a non-conventional manner, such as withoutdielectric over the electrodes in an alternating current (“AC”) mode. Inone embodiment, protective dielectric layer 30 is extremely thin (e.g.,less than or on the order of 0.01 μm) so as to better allowphotoelectrons emitted by photocathode 26 to reach gas 34. In thisembodiment, dielectric layer 30 may be too thin to act as a fullyfunctional insulator between photocathode 26 and X-electrodes 22, anddielectric breakdown might occur in the narrow overlap region betweenphotocathode 26 and under the X-electrode if the voltage differencebetween these two layers becomes significant (e.g., greater than about 1volt). However the possibility of some dielectric breakdown betweenthese layers in one embodiment is not problematic and in fact may bebeneficial because the breakdown will short out any significantpotential difference between the layers and thus re-establish andmaintain the desired low value which most likely would be close to azero voltage bias. A dielectric breakdown may likely not occur in someembodiments, as many semiconductor devices (e.g., transistors builtusing a 90 nm CMOS process) employ an even thinner 1.2 nm insulationlayer for the gate dielectric and are only about 5 silicon atoms thick.In these known devices, the transistor gate dielectric is typicallyformed from SiO₂ which is typically used with a bias on the order ofabout one volt, thus the gate dielectric requirements for SiO₂ are morestringent than those anticipated for protective dielectric layer 30, asdisclosed below.

In one embodiment, the potential difference or bias between the X- andY-electrodes 22 and 24 (e.g., cathode and anode for device 10 in a DCconfiguration), is in the general range of 300 to 1000 volts dependingupon specific device design and materials (i.e., including such keyparameters as gas composition, gas pressure, discharge gap, electroderesolution, etc.). In one embodiment, the configuration and materialsused for back substrate 18 can generally mirror those chosen for frontsubstrate 16. In one embodiment, protective dielectric layer 30 isformed from a coating thickness on the order of 4 nm of the dielectricCsBr (dielectric constant of 6.5 as compared to 4.5 for SiO₂) andprotects photocathode layer 26, which can be formed from CsI, Cs₂Te,K₂CsSb or any one of a number of various other photocathodic materials.Since Cs has about twice the atomic diameter of Si, and the Sidielectric works at a thickness of 1.2 nm, a Cs based dielectric can beeffective at a thickness of about 2.5 nm, and so a 4 nm thick layershould function as a reasonable insulator.

In one embodiment, the position of photocathode layer 26 on the frontsubstrate 16 (transmissive mode) can be moved to the back substrate(reflective mode) and would replace conductive layer 28 which would nolonger be needed. In this embodiment, “ultra-thin” protective dielectric30 on the front substrate would also be moved to the back substrate tosimilarly replace electrode insulation layer 32. As a result of thesechanges, Y-electrodes 24 on the back substrate (in this reversedphotocathode configuration) function as the cathode with typically onlyabout one volt of potential bias difference between the back substratephotocathode and the Y-electrodes. In order to enhance the transmissionefficiency of photons from scintillation plate 12 reaching thephotocathode, which is now on the back substrate, X-electrodes 22 on thefront substrate should be made as transparent as possible (e.g., ITO orSnO).

PPPS detector 10 of FIG. 1, and the additional PPPS detector embodimentsdisclosed below, operate as highly-pixelated radiation detectors withtheir pixels briefly turning “on” (i.e., they are normally “off”) andthereby being counted in direct proportion to incoming radiation, and soat their most basic level function as digital radiation counters withthe addition of counting circuitry disclosed below. Further, since thesedetectors do not operate in the proportional region, each such pixel gasdischarge pulse or event is considered to have approximately an equaldischarge value (i.e., current) and from an information processing ordetection point of view is simply counted as a single event. Therefore,embodiments of the present invention are not considered proportionaldetectors.

In embodiments of the present invention, all radiation induced pixeldischarges begin (i.e., are turned “on”) by initially maintaining thepanel voltage just below its spontaneous discharge setting, such thatany free-electron upon entering the gas can quickly set off a dischargeat the nearest pixel site which can very rapidly (e.g., estimatedresponse time on the order of about 10 ps) grow into a localizedhigh-gain avalanche. In accordance with some embodiments, thefree-electron is generated via a two-step process in which the incidentradiation first interacts with a top scintillator “plate” (or crystal,or other such top luminescent material layer or coating) such asscintillation plate 12 of FIG. 1, to emit “optical” photons that caninteract with the photocathode layer within the PPPS, which in-turnemits photoelectrons (i.e., free-electrons) into the gas. The actualpixel sensing and counting can be performed by circuitry such as digital(i.e., photon-counting) acquisition electronics to store time-taggedpixel discharge information and correlated X-Y events.

PPPS detector 10 in accordance with embodiments of the present inventionis generally a direct, high-gain, position and intensity sensitive,digital counter/detector of “optical” photons. A single solitaryphotoelectron (i.e., free-electron) upon entering the high field, highresolution, pixel space of a suitably designed plasma panel cell, can,in accordance with the various embodiments of the present invention,experience almost instantaneous internal electron amplification with again of approximately eleven orders-of-magnitude, without externalamplification and without loss of spatial resolution. The resultingelectron avalanche, which can be both confined and self-contained withinthe region that defines each pixel's cell space, generally occurs on thesub-nanosecond time scale. The incident flux or intensity sensitivity ismeasured by the number of cells firing repeatedly. Given the extremelyfast pixel response time and therefore the high counting rate capabilityof the PPPS, coupled with the dense pixel structure, the total number ofdischarging cells should be proportional to the incident radiation flux.However, the number of “adjacent” or “near-adjacent” pixels discharging“simultaneously” should correlate to the number of photons generated inthe scintillation plate by a single “hit” and hence theenergy-originally deposited by the incident photon or particle. Thus thetotal number of gas discharge events within a tight grouping (bothspatially and temporally) is proportional to the incident photon orparticle energy that originally gave rise to this burst of dischargeevents, whereas the total number of such groupings or bursts isproportional to the incident radiation flux.

In some embodiments of the invention, as disclosed below, the PPPSdetector does not require the use of scintillation plates or crystals.Similarly, most embodiments do not require the use of high-pressure,expensive, high-purity isotopic gases such as ³He or ¹⁰BF₃, althoughsuch gases might be useful for certain applications.

FIG. 2 is a perspective view of a surface-discharge PPPS detector 300 inaccordance with one embodiment of the present invention. In oneembodiment, detector 300 operates in the DC mode, but as with device 10of FIG. 1, it could be adapted to operate in the AC mode. Detector 300includes a front substrate 310 and a back substrate 312 separated by agas-filled gap 314. Detector 300 further includes a photocathode 350facing discharge gas 316. In one embodiment, a scintillation plate maybe used as front substrate 310 in which case an external opticalcoupling layer (not shown) would not be needed. In another embodiment, ascintillation plate may be added and optically coupled to frontsubstrate 310 in a manner analogous to that disclosed in FIG. 1.

Radiation detector 300 further includes surface-discharge X-electrodes320 and Y-electrodes 324, which together define a pixel structure thatin response to an emitted electron from photocathode 350 can cause asurface-discharge shaped avalanche across gas-discharge gap 318resulting in a gas-discharge counting event, disclosed below. Detector300 further includes a current-limiting series resistor 340 (i.e.,impedance) on each surface-discharge electrode in each discharge cell.Detector 300 may further include a back substrate dielectric layer 344which is used as an insulating layer in one embodiment where detector300 includes orthogonally oriented, auxiliary Z-electrodes 328 forlocating the orthogonal position of a surface discharging X-Y pixel. Inone embodiment, an ultra-thin protective dielectric layer 360 is used toprotect photocathode 350 from ion bombardment and can be formed aspreviously disclosed in conjunction with FIG. 1 from a coating thicknesson the order of 10 nm or less of a suitable dielectric such as CsBr. Inone embodiment, Z-electrodes 328 are deposited directly on top of backsubstrate 312 but beneath dielectric layer 344. The Z-electrodes in theabove embodiments are not gas-discharge electrodes but are low-current“auxiliary” electrodes that can assist in sensing the location of ahigh-current avalanche event between X-Y surface-discharge electrodes320 and 324. Further, in one embodiment detector 300 includes thetransparent conductor 354 (e.g., ITO or SnO) to enhance the electricalperformance (e.g. conductivity) of photocathode layer 350 should it beneeded. Further, in one embodiment, if charge accumulation on the toplayer of the portion of back substrate 312 which faces gas 316 is deemeda problem, then Z-electrodes 328 can be used to drain this charge, or inan embodiment that does not include Z-electrodes, a conductive coating,similar to layer 28 in FIG. 1 or layer 354 in FIG. 2, could be depositedon back substrate 312. As previously disclosed in conjunction withdetector 10, in one embodiment of the above surface-discharge PPPSdetector the positions of the photocathode and electrodes can bereversed, along with associated electrode circuitry (e.g., resistors),dielectric layers, etc. For this reversed surface-discharge PPPSembodiment the photocathode operates in the reflective mode on the backsubstrate instead of the transmissive mode on the front substrate.Further, to enhance the transmission efficiency of “optical” photonsreaching a back substrate photocathode, the electrodes on the frontsubstrate should be made as transparent as possible (e.g., ITO or SnO).

FIGS. 1 and 2 illustrate the interior cross-sectional structure of PPPSdetectors 10 and 300. In addition, embodiments of these detectorsfurther include a hermetic seal (not shown) to contain the dischargegas. In one embodiment, the hermetic seal is along the entire outerperimeter or is an external structure that surrounds the detectors.Numerous spacers may be included inside the seal to maintain the correctgas gap.

In one embodiment, PPPS detectors 10 and 300 include a barrier structurethat is located between the two substrates for the purpose of physicallyand electrically isolating each pixel or gas-discharge cell. FIG. 3 is aperspective view of one such barrier 50 in accordance with oneembodiment that can be used with the disclosed detectors. Barrier 50includes rectangular walls that surround each pixel cell in asymmetrical fashion with the barrier opening centered with respect tothe center of each pixel discharge site. However many other barrierstructure configurations can be used, including shapes that could bedescribed as diamond-patterned, honeycomb, egg crate, cylindrical,triangular, saddleback-shaped, shadow masks, etc. These various shapedbarrier structures, as well as numerous others designed for similarpurposes to physically and electrically isolate individual pixels orgroupings of such pixels in plasma display panels (“PDP's”), are knownstructures that have been developed over many years for the TV-setindustry.

In one embodiment, PPPS detectors 10 and 300 and other embodimentsdisclosed below may be structured to function as either AC or DCdevices. In one embodiment, the DC PPPS detector is configured as a“columnar-discharge” detector as shown in FIG. 1 structured with a“bare” cathode (X-electrodes 22) facing a “bare” anode (Y-electrodes 24)with no dielectric layer in between the electrodes and separated by agap 20 that is filled with discharge gas 34. In this embodiment, the DCdetector is normally kept at a constant “ready-to-discharge” voltage viadirect connection to a steady, well-regulated DC power supply circuit.In contrast, for the AC detectors, the discharge electrodes constantlycycle back and forth between two effectively opposite voltage plateausand are therefore in a receptive (i.e., ready-to-discharge) state foronly a part of each cycle. For these transition periods during which thevoltage is changing, the device will be unresponsive (i.e., experiencedead-time) and any radiation generated free-electrons that manage tofind their way into the gas will essentially be “lost” (i.e., notcounted). In addition, all AC PPPS detectors (with or without thescintillation plate 12 and optical coupling layer 14) with aconventional (i.e., charge storing) dielectric layer over the X- andY-electrodes include significantly complex drive-waveforms incorporatingdielectric wall-charge erase functions for neutralizing accumulatedcharge stored from previously lit “on” pixels. Further, commercial ACplasma display panels (“PDP's”) typically have a thick-film dielectriccoating over the electrodes greater than 10 μm, and often 20 to 40 μm,which is much too thick to efficiently allow any photoelectronsgenerated by a photocathode layer underneath such a dielectric passthrough and reach the gas. For this reason, AC PPPS devices in oneembodiment do not include the columnar-electrode structure as shown FIG.1 if the device incorporates a thick-film dielectric layer over theX-electrode. However if there is essentially no dielectric overX-electrode 22 as shown in FIG. 1, then a “bare-electrode”columnar-discharge AC PPPS detector can be implemented.

AC PPPS detectors in accordance with other embodiments can also befabricated based on surface-discharge electrode structures such as shownfor the PPPS detector of FIG. 2, with the X- and Y-electrodes both onone substrate and the photocathode on the other. For theseconfigurations, a conventional dielectric layer (e.g., 10 to 40 μmthick) can be employed over the two discharge electrodes (i.e., X and Y)because this thick dielectric layer would not be in contact with thephotocathode. In another embodiment, a top thin-film MgO refractorycoating (e.g., 0.1 to 1 μm thick), such as that employed for commercial“PDP's”, can be applied either directly over the X- and Y-electrodes orover the above-described conventional (i.e., thick) dielectric on top ofthese two electrodes to provide a more stable and improvedsputter-resistant electrode surface. The AC PPPS thin-film overcoatlayer (e.g., MgO) might be further improved in terms of its probabilityof directly interacting with ionizing radiation (i.e., via absorptionand/or inelastic scattering), by replacing the low-Z (i.e. “Z” as usedhere being the atomic number according to convention), “standard PDP”emissive MgO coating with a higher-Z emissive coating such as La₂O₃,Eu₂O₃, etc. However all such candidate emissive thin-film overcoatlayers should, like MgO, be chemically and thermally stable,sputter-resistant, and thermally activated at process-compatibletemperatures.

One further advantage in replacing the “standard” MgO coating with adifferent thin-film dielectric material is that MgO promotes“exoemission” of free-electrons (i.e., exoelectrons) into the gas in ACPDP devices. The mechanism of exoemission, also called relaxationelectron emission, can proceed via a multitude of pathways, but mostcommonly involves the release and transfer of energy from trapped chargecarriers to an electron that can escape into the gas. This “spontaneous”electron emission process from a dielectric can occur a long time afterthe original excitation, from seconds to months, and can therefore be asource of background noise in an AC (or even a DC) PPPS detector. Oneway to reduce the impact of such background noise is by reducing thesurface area of all dielectrics, and by minimizing dielectric defectsand strains in the manufacturing process (e.g., annealing). In anotherembodiment, the AC PPPS detector can be employed without any type ofcharge-storing dielectric layer. In one embodiment, thesurface-discharge AC PPPS detector could look identical to the DC PPPSstructure shown in FIG. 2. Depending upon the application, thesurface-discharge structure in FIG. 2 could be used with or without ascintillation plate and optical coupling layer (e.g., as shown in FIG.1). Yet even in DC PPPS detectors, the X and Y discharge electrodesshould still be on top of and separated by a dielectric surface whichinterfaces with the gas, so some level of spontaneous exoemission (i.e.,electronic noise) will generally always occur.

In one embodiment, to enhance the positional or angular resolution of aPPPS scintillation detector, the uncertainty of the reaction sitelocation for a particular radiation absorbing or scattering event in a“thick” scintillation plate or crystal can be reduced byvertically-stacking an “equivalent” number of “thinner” scintillationplates, each optically coupled to a PPPS. FIG. 4 is a perspective viewof a vertically-stacked PPPS scintillation detector in accordance withone embodiment that includes stacked thin scintillation plates 62-65.The thinner the scintillation plate, the greater the number ofvertically-stacked PPPS-scintillation detectors required to achieve agiven level of radiation interaction (e.g., absorption or scattering).However, the thinner the scintillation plate, the smaller theuncertainty with respect to reaction site location, hence the better theoverall positional and/or angular resolution of the integratedPPPS-scintillation detection system. Therefore, to achieve improvedpositional and/or angular resolution in this type of vertical-stackeddetection system requires thin, flat panel, PPPS devices. In general,the greater the level of stacking (i.e., number of stacked plates) thebetter the positional/angular resolution, but also the larger the areaof PPPS devices needed. Thus only a low cost detector technology such asthe PPPS can be fully-exploited for such configurations as most otherflat radiation detectors (e.g., semiconductor based devices) would notbe affordable. The extra degree of freedom associated withvertical-stacking also allows for a variety of innovative hybridstructures, such as having different spectral response optimized deviceson top of each other, or integrating gamma-ray detectors with neutrondetectors, etc.

FIG. 5 is a block diagram of a vertically-stacked PPPS-scintillationdetector 70 in accordance with another embodiment of the presentinvention. Detector 70 represents a Compton telescope arrangement thatcan exploit the physics of tracking multiple sequential Comptonscattering events. Some advantages of the Compton telescopeconfiguration of FIG. 5 is the ability to improve system efficiency,spectroscopic energy resolution and angular resolution by in part theelimination of collimation optics, although other system configurationssuch as coded-apertures could also achieve some of these benefits with adifferent set of trade-offs using the PPPS-scintillation detector. Inone embodiment, detector 70 utilizes a technique known as the “3-Comptonmethod”.

For all PPPS gas discharge type detectors disclosed above (both AC andDC), a simple relationship known as the Paschen curve gives the firingor breakdown voltage as a function of the product of gas pressure anddischarge gap. Based on this classical relationship, PPPS internal gaspressure should be increased as the device pixel pitch decreases. Forvery high pixel resolutions, it could thus be advantageous to increasethe internal panel gas pressure above one atmosphere. However, from amechanical design viewpoint, it may be difficult to maintain a uniformgas gap in a plasma panel such as that shown in FIG. 1, having a totalthickness of only about 1 mm while holding a positive internal gaspressure. In one embodiment, the scintillation plate/crystal materialsof the detectors, such as plate 12 of FIG. 1, are hydroscopic andtherefore have an encapsulation package or mechanical housing tomaintain an inert or dry atmosphere. In this embodiment, the gasatmosphere within the mechanical housing may be adjusted such that itapproximately matches the PPPS positive gas pressure, therebyeliminating any significant pressure differential acting upon the PPPSstructure. Thus by utilizing an integrated housing system design, thePPPS should be able to maintain a uniform gas gap in a positive pressuredevice, regardless of the particular configuration or how thin thedevice might be. In other embodiments, instead of using a conventionaltwo plate device structure such as that shown in FIGS. 1 and 2, somePPPS configurations can internally contain a positive gas pressurewithin a fully-encapsulated, closed-cell, microcavity-based, gasdischarge pixel structure as disclosed below.

FIG. 6 is a cross-sectional view of a microcavity-PPPS detector 100configured to operate as a DC device in accordance with one embodimentof the present invention. Detector 100 includes a front substrate 102which is transparent (e.g., glass) to the incident radiation ofinterest. Detector 100 further includes a photocathode layer 104deposited on the inside surface of substrate 102 and in contact with thePPPS gas, which in-turn can form a gas discharge 114 between an anode108 and a cathode 112. In one embodiment, an ultra-thin protectivedielectric layer (such as CsBr disclosed above) is applied overphotocathode 104. In another embodiment a transparent conductive layeris deposited between photocathode 104 and front substrate 102 to enhancethe photocathode electrical properties (e.g., conductivity). Cathode 112has an inverted pyramid structure that can be formed, for example, bythe wet-etching of a silicon-based substrate such as p-Si, although anymaterial that can be suitably fabricated to form a microcavity typestructure can be used.

In order to electrically isolate anode 108 from cathode 112, adielectric insulation layer 110 is deposited that can be patterned(e.g., by photolithography) as shown in FIG. 6. In one embodiment,dielectric layer 110 has a thickness between a few microns to tens ofmicrons thick and is fabricated from a material compatible with thecathode substrate—e.g., silicon nitride (Si₃N₄), silicon dioxide (SiO₂),etc. In one embodiment, front substrate 102 and back substrate 112 canbe electrically isolated via a dielectric spacer layer 106 which can bedeposited on top of anode 108. The anode material should be fairlysputter-resistant. An example of one such material is nickel with atypical thickness of about 1 μm. In order to fabricate amicrocavity-PPPS-scintillation detector, a scintillation plate andoptical coupling layer, as shown in FIG. 1, is added to front substrate102. In addition to having the capability of operating at a positive gaspressure, a second advantage of the microcavity-PPPS is being able toattain a fully-encapsulated, self-contained, gas-discharge without thecomplication of having to fabricate an internal barrier structure. Otherembodiments containing different shaped microcavities also exist,including such geometries that are cylindrical, spherical,hemispherical, conical, etc.

In one embodiment, photon-counting (i.e., pulse detecting) typeelectrode circuitry for detecting each gas discharge cell interactionand counting each such interaction as an individual pixel dischargeevent is coupled to the PPPS detectors disclosed above. FIG. 7 is ablock diagram of a counting circuit 80 in accordance with oneembodiment. In FIG. 7, X- and Y-electrodes, 81 and 82, are shown asbeing in an orthogonal arrangement (i.e., rows and columns) such as thatshown in FIG. 1. However the X- and Y-electrodes are not restricted toorthogonal patterns and can be configured in any such way consistentwith being able to count and record pixel discharge events. For example,one alternative X-Y configuration is shown in FIG. 2, in which the X andY discharge electrodes are patterned in a surface-discharge typeparallel arrangement, but can still employ electrode readout circuitrysuch as that shown in FIG. 7. Another example is the configuration shownin FIG. 6. Further, the counting circuitry used with the PPS, disclosedin U.S. patent application Ser. No. 11/155,660, the disclosure of whichis herein incorporated by reference, may also be used with embodimentsof the present invention. The circuitry disclosed above can utilize anyone of a number of current-limiting impedance component arrangementswith regard to the two, gas-discharge electrodes (e.g., variouslydescribed as X and Y, or cathode and anode, or row and column), whichare in-turn coupled to the event counting detection electronics shownin, for example, FIG. 7.

The various embodiments of the detectors of the present invention asdisclosed herein, and as illustrated by the circuitry shown in FIG. 7,are fundamentally digital in nature and as such the detectionelectronics does not record the magnitude of a given cell discharge asdoes most “conventional” detectors operating in the linear region asproportional detectors, but instead operates in the non-linear regionand can employ Geiger-Mueller type counting techniques/circuitry, thusassigning essentially the same value to each event regardless of thecell discharge magnitude. Embodiments of the present invention can usecircuitry to acquire pixel discharge data by utilizing standard pointscanning, line scanning, or area scanning techniques.

Circuit 80 further includes a discriminator 84 to produce logic pulseswhich can then be fed to an array of field-programmable gate array(“FPGA”) logic arrays 86. FPGA arrays 86 can perform the calculation ofthe position for each hit, and emit a stream of time-stamped (X,Y)coordinates. For the embodiment of the PPPS-scintillation detector shownfor example in FIG. 1, a radiation hit or interaction within thescintillator plate should generally cause a shower of photons andsubsequent photoelectrons to enter the space of the immediate nearbycells and thus initiate numerous pixel discharges resulting in aplurality of counting events. Discriminators 84 must therefore be ableto identify multiple hits on each electrode, and send this countinformation to FPGA 86.

In one embodiment, to accurately record the number of hits on a wire ifthey get too large, the readout electronics are organized via a gridtype of architecture that monitors, records and integrates theindividual event counting results from a number of smaller sub arrays,thus requiring that more wires be brought out to reduce the number ofcoincident events along an extended length of electrode wire. Bringingout more wires require more, but simpler, discriminators.

In terms of avalanche control and cell discharge characteristics, thecircuit performance can generally be enhanced by the addition of anappropriate current-limiting series resistor(s) and/or possibly othersupport circuitry within each cell, or within a grouping of such cells,such as shown in FIGS. 2 and 7. Such resistor configurations orgroupings can serve to decouple neighboring pixels from each other, thuslimiting or quenching the discharge before the avalanche can spread fromone pixel to another. For example, a resistor can be used in each cellor in a string or grouping of cells for an electrode wire, with suchelectrode wires groupings configured to form a sub array within a largerPPPS array structure. Fabrication of the internal cell circuitry cannormally be accomplished most economically by utilizing thick-filmand/or thin-film deposited components (e.g. conductors, resistors,dielectrics, etc.), possibly in combination with thin-film amorphous (orlow temperature polycrystalline) semiconductor multilayer circuitfabrication technology, such as that employed in the production ofTFT-LCD's, etc. Such multilayer circuit design configurations canincorporate pixel I/O wires as well as circuit components such asresistors, diodes, insulators, etc., which can be fabricated and locatedinside, alongside or beneath the active pixel (i.e., discharge cell)surface structure or in a suitable location nearby. For the readoutelectronics of FIG. 7, the more complex system signal processingelements, which can include field programmable gate array (“FPGA”) logicdevices and associated discriminators, can be implemented with discreteor integrated components positioned along the panel periphery of thePPPS using, for example, chip-on-glass or chip-on-flex-circuittechnology similar to that employed for connecting PDP driver chips toeach electrode on a PDP glass substrate.

Embodiments of the present invention can be used for monitoring agentsof bioterrorism and biowarfare, and can involve live-cell imaging and/oranalysis, as well as molecular genetics, and can employ syntheticfluorophores, fluorescent proteins, immunofluorescence reagents andother intracellular derivatives in probing subcellular structures foruse in DNA sequencing, in-vitro assays, chromosome analysis, geneticmapping, etc. Further, embodiments can be used for bioluminescentdetection of bacterial contamination in the environment, including food,water and air. In these embodiments, the PPPS detector can be designedto function as a high resolution, high sensitivity, imaging device, orconversely as a non-imaging, large-area photosensor that can be tuned todetect specific wavelength emissions from “tagged” fluorescent species.In the latter embodiment, where X-Y positional sensitivity and imagingcapability are not needed, the detection electronics can be greatlysimplified. In one such embodiment, many if not all of the X-electrodescan be shorted together and connected to a single series resistor, ascan be done for the Y-electrodes. For such embodiments, detectionsensitivity would rely on the extremely fast, sub-nanosecond pixelresponse times discussed above in conjunction with detector 10. Forexample, if all X-electrodes were shorted through a single resistor, aswere all Y-electrodes, then the above PPPS detector could reset itselfeach time a pixel discharge event occurred. If the detector dischargeand reset time were 1 ns, then as many as 10⁹ events per second could bedetected.

Numerous known methods exist for exciting tagged fluorescentchromophores including: UV-VIS light sources in combination withdichroic mirrors and filters, tunable lasers, fluorescence resonanceenergy transfer or FRET (also called Förster resonance energy transferor dipole-dipole resonance energy transfer), and total internalreflectance fluorescence (“TIRF”) with surface evanescent-wave coupling.

FIG. 8 is a cross-sectional view of a PPPS-TIRF cell 200 in accordancewith one embodiment for collecting fluorescent photons 214, excited viathe evanescent-wave from an external illumination source 206 (e.g.,laser), propagating along an optical fiber waveguide 202 (or matrix ofsuch fibers or an optical plate), by means of total internal reflection(T.I.R). The evanescent-wave along the T.I.R. waveguide 202 canstimulate fluorescence from select molecules at or very near the surfaceinterface with a fluid media 208. Fluid media 208, which may be liquidor gas, can be either static or dynamic in motion. In one embodiment,PPPS-TIRF cell 200 includes input 210 and output 212 circulationplumbing to allow fluid media 208 to flow through the cell cavity forsampling a much larger media volume than if the fluid sample volume wereconfined to the fixed dimensions of a static PPPS-TIRF cell design. Forsome embodiments, fluid media 208 is suitably transparent to thefluorescent photons 214 emitted at or near the coated surface layer 204of the T.I.R. waveguide 202. This allows emitted photons 214 to reachand be counted by the PPPS-detector 216. Possible configurations for theT.I.R. waveguide 202 are those based on an optically flat platestructure such as a luminescent concentrator or a matrix of opticalfibers.

Embodiments of cell 200 of FIG. 8 can be used for a number of differentapplications by custom preparation of the sensitized coated surfacelayer 204. For example, an evanescent-wave biosensor for anantibody-based detection system for detection of biothreat agents can beadapted to PPPS-TIRF cell 200 to achieve improved biosensor sensitivityat reduced cost. FIG. 9 is a cross-sectional view of a PPPS-TIRF dualcell 250 in accordance with one embodiment. Cell 250 allows for thecollection of the evanescent-wave fluorescence from both sides of theT.I.R. waveguide and can improve the efficiency of the cell. Forexample, for the evanescent-wave biosensor for an antibody-baseddetection system, capture antibodies can be coated either directly orindirectly (e.g., via a polymer such as polystyrene or other appropriatematrix) on the T.I.R. waveguide to form a sensitized coated surfacelayer 204. The sample fluid media would then be passed over the T.I.R.waveguide(s) with target analytes captured by the antibodies.Fluorophore-labeled reporter antibodies are then passed through thesystem and should bind to the captured target analytes on thewaveguides. Fluorescent reporter molecules within approximately 1 μm ofthe waveguide surface would be expected to be excited by the evanescentfield of the excitation source (e.g. laser), with many of the resultingfluorescent photons captured and recorded by the PPPS-detector. Becauseof the large area, high gain, direct-digital photon counting capabilityof the PPPS-TIRF cell of FIG. 8 or 9, very large detection surfaces arefeasible for numerous applications requiring high sensitivity at lowcost.

Other embodiments of the present invention can be used for additionalbiotechnology applications. For example, instead of the TIRF methodologybased on surface evanescent-wave coupling shown in FIGS. 8 and 9, adirect excitation of tagged fluorescent chromophores in the sample fluidmedia via an electronically modulated (e.g., shuttered, phase-locked,etc.) illumination source may be used. In another embodiment, anappropriately fast pulsed-laser is used so that the PPPS-detector withsuitable discriminating, filtering and/or buffer circuitry would only be“looking at” the emitted, time-separated, fluorescence photons, andtherefore not “see” the illumination source itself. Alternatively thePPPS-detector could employ an optical blocking filter directly on top ofPPPS front plate 218 in FIG. 8 to prevent direct entry of the incidentillumination signal and thus only be sensitive to the spectrallydown-shifted fluorescence signal. A similar but slightly modifiedembodiment employs a narrowband transmission filter on top of PPPS frontplate 218 to allow only photons having a specific wavelength of interestto be passed through the filter and be detected by the PPPS. Theseembodiments can be used, for example, for the analysis of foodstuffs(e.g., grains, vegetables, fruits, dairy and meat), powders, clinicalspecimens, environmental air samples, drinking water, irrigation water,wastewater, etc., for harmful compounds such as toxins, microbes,bacteria and viruses.

PPPS detectors in accordance with one embodiment include quantum-dotmaterials that function as photocathodes. For these embodiments, thedetectors can be constructed without a “conventional” photocathode bycoating (e.g., spin-coating) the same inside substrate surface thatwould normally support a conventional photocathode with an appropriateorganic quantum-dot producing material and then “burning-out” theorganic carrier leaving behind a “pure”, photoemissive, thin-film,quantum-dot layer—i.e., a photocathode surface. In another embodiment,the PPPS detector includes such materials as quantum-dot fluorophores orscintillators in order to take advantage of their potentially very highefficiency and/or very fast response times as compared to conventionalfluorophores.

Embodiments of PPPS detectors can be implemented as direct-PPPSconversion devices, or photodetectors, without use of either internalconversion layers or photocathodes by constructing a PPPS detectorutilizing a special “VUV scintillator plate” as the front substrate thatcan emit VUV (i.e., vacuum ultraviolet) photons into a suitable plasmapanel discharge gas that can be ionized upon absorption of a VUV photon,thereby causing a local cell avalanche. Possible host substrates forsuch a scintillator plate include LiF and MgF₂ which primarily have thevirtue of being the two most transmissive “optical” materials for theVUV “optical” region (i.e. transmission down to ˜100 nm and 120 nmrespectively). However, selection of an actual VUV phosphor that mightwork with either of these two host crystals (e.g., LiF, MgF₂, etc.) willdepend not only on the host crystal field strength, etc., but also onthe form of incident radiation—e.g. gamma-rays versus ionizingparticles. The most widely used VUV scintillator for gamma radiation isBaF₂ (due in part to its high-Z barium component), which although not astransparent as LiF or MgF₂, does transmit down to about 140 nm withsignificant VUV emission at slightly longer wavelengths (but still below200 nm).

In addition to having a satisfactory VUV scintillator, embodiments ofthe direct-PPPS detectors must also include a suitable discharge gascapable of being ionized by photons emitted from the VUV scintillator.Some examples of relatively low ionization constant inorganic gases thatcan be used for such an application, as well as for the various PPPSdevices disclosed above, either by themselves or in combination withother gases are: Hg (10.4 eV), NH₃ (10.1 eV), NO₂ (9.6 eV), and NO (9.3eV). Examples of suitable low ionization constant organic gases include:ethylene C₂H₄ (10.5 eV), propyne (or methylacetylene) C₃H₄ (10.4 eV),dimethyl ether C₂H₆O (10.0 eV), propene C₃H₆ (9.7 eV), 1,2-propadieneC₃H₄ (9.7 eV), methylamine CH₅N (8.9 eV), dimethylamine C₂H₇N (8.2 eV),trimethylamine C₃H₉N (7.9 eV), etc.

Some host gases that could be used in conjunction with, or in additionto, those disclosed above, are the same types of gases that would“normally” be used for all of the previously discussed PPPS devicesdisclosed herein, and may include the following either by themselves orin combination: Ar, N₂, Xe, Kr, CH₄, CF₄, C₂H₆, etc. For example, twocommercially available “Proportional Counting Gases” sold as P-5 andP-10 gas are respectively 95% argon/5% methane, and 90% argon/10%methane. Another common radiation detector gas mixture is 95% argon/5%nitrogen, as well as numerous combinations of the above mixtures, withand without various avalanche quenching agents such as propane, butane,etc.

Embodiments of detectors disclosed above can be used for a variety ofapplications involving the detection of ionizing particles. Suchapplications include detecting low and high energy (i.e., slow and fast)neutrons as well as other ionizing particles for which prior artmicropattern detectors have typically been used. Two significantapplications for embodiments of the present invention include thedetection of neutrons emitted: (1) by special nuclear materials (“SNM”)of serious concern regarding weapons of mass destruction (e.g., uraniumand plutonium), and (2) in particle accelerators for high-energy andnuclear physics. In terms of nuclear accelerators, in addition todetecting emitted neutrons, embodiments can provide for the detection ofcharged particles for radioactive ion beam (“RIB”) profile diagnostics.In detecting charged particles for RIB-profile diagnostics, dependingupon the beam energy, etc., embodiments may not need either aphotocathode or conversion layer as charged particles passing throughthe cell's active gas volume will create free-electrons by collisionswith the gas atoms. This is particularly true in the realm of higherenergy charged particles in the MeV to GeV range, in which suchparticles would have little difficulty in passing through the detectorfront substrate. In terms of detecting lower energy charged particles,the front substrate could be made extremely thin (i.e., on the order of0.1 mm, or less) and the device materials and structure modifiedaccordingly (e.g., using low density materials) per standard practiceknown to those skilled in the art to which the present inventionpertains.

Embodiments of the present invention can internally discriminateneutrons from gamma-rays, in part, due to the highly-pixelated,open-cell structure of the device as configured for neutrondetection—i.e., high pixel resolution without internal cell barriers.This would be in addition to enhancements gained from reduced gamma-raysensitivity through use of thin, low density, low-Z substrate materials,external gamma-ray filters (e.g., lead), and other “standard” software,firmware and electronic discrimination techniques known to those skilledin the art. More specifically, one embodiment utilizes asurface-discharge PPS type structure 400 modified for neutron detectionas shown in FIG. 10. This structure is similar to the surface-dischargePPPS device shown in FIG. 2, but with the transparent conductor 354,photocathode layer 350, and protective dielectric layer 360 alleliminated, and replaced with a neutron capture and alpha-particleemitter layer 410 facing the discharge gas. An example of one suchsuitable material for layer 410 is boron. Absorption or capture of aneutron by ¹⁰B in the above PPS detector 400 would typically be followedby emission of an alpha-particle (or excited ⁷Li particle) which canresult in a trace or string of free-electrons being created along theemitted alpha-particle's path in the gas 420. If, for example, the gasgap separation layer 430 between the disclosed PPS neutron detectorfront and back substrate is 0.4 mm, and the pixel pitch is 0.01 mm, thenan alpha-particle moving at an average angle of 45 degrees with respectto the substrate and with an energy on the order of 2 MeV could excite aone-dimensional string of about 50 pixels to turn “ON” (i.e. discharge).On the other hand, an incident gamma-ray interacting through themechanism of Compton scattering can also result in a series of pixeldischarges, but the latter would not be produced in the linear tracepattern characteristic of an emitted alpha-particle. Thus the uniquecombination of an open-cell PPS pixel structure of one embodiment beingable to generate a linear string of free-electrons in the gas, coupledwith the fine pixel pitch of the PPS neutron detector that can resolvesuch a linear trace of excited free-electrons each of which can cause alocalized pixel discharge event, results in a unique neutron particledischarge signature for the above PPS device. In general, such a PPSneutron detector in accordance with one embodiment could be thought ofas a high resolution cloud chamber in terms of actually “seeing” theresulting alpha-particle (or excited ⁷Li particle) trace of excitedpixels through the PPS detector, which would be significantly differentfrom the pattern produced by isolated free-electrons resulting fromCompton scattered gamma radiation.

The efficiency of embodiments of the PPS neutron detector should be goodwith respect to slow (i.e., thermalized) neutrons, which can be capturedby the ¹⁰B containing layer 410 due to its high neutron cross-section.To capture fast neutrons, emitted for example by special nuclearmaterials (“SNM”), one embodiment utilizes a hydrocarbon moderator suchas polyethylene in front of the PPS to thermalize the incident neutronradiation. In another embodiment the neutron detection efficiency can beimproved by configuring the detection system in a vertical stackarrangement (which could also be laminated) such as that in FIG. 4, inwhich PPS neutron detectors 400 are substituted for the PPPS devices(i.e., PPPS-1 through PPPS-4) shown in FIG. 4. By using thin film layersof ¹⁰B, the probability of alpha-particle emission in the gas from agiven film layer upon neutron capture can be made relatively high, andeven if layered to be a hundred devices deep for greater neutronabsorption the total detector vertical stack might still be only a fewinches thick. With its large potential detection area and very highpixel spatial resolution, and vertically stacked for enhancedefficiency, such systems could be made to function somewhat similar to a“solid state” cloud chamber, but with greater capability and at muchlower cost. In other embodiments, different materials can be employedfor layer 410. For example, some materials that can be substituted for¹⁰B which have good neutron capture cross-sections include ⁶Li and¹⁵⁷Gd.

A number of embodiments are specifically described herein. It will beappreciated however, that modifications and variations of these arecovered by the above teachings and therefore fall within the purview ofthe appended claims without departing from the spirit and intended scopeof the invention.

For example, in additional embodiments, the X- and Y-electrodes can takeon a variety of different shapes from those disclosed above, butotherwise operate in similar fashion. For example, the X-Y electrodestructure and X-Y pixel series resistor configuration shown forsurface-discharge PPPS detector 300 of FIG. 2 can be modified to take ona significantly different pixel appearance and different impedanceelectronic design, yet operate in a similar manner. One such embodimentis shown in FIG. 11 for PPPS detector 500, where X and Ysurface-discharge electrodes 520 and 524 consist of two concentriccircles that form “a repeating bull's-eye pixel pattern”. In thisembodiment, each of the “interior” surface-disk electrodes (i.e., anX-electrode) has one series resistor 540 per individual pixel cell, butalso just one series resistor 530 per corresponding “exterior” columnline (i.e., a Y-electrode). Depending upon the application, differentembodiments based on the many variations in device structure, etc. asdisclosed above will each have their own set of advantages anddisadvantages attributed to their respective differences in design.

1. A radiation counting detector comprising: a first substrate; a secondsubstrate generally parallel to said first substrate and forming a gapwith said first substrate; a gas contained within said gap; aphotocathode layer coupled to said first substrate and facing saidsecond substrate; at least one first electrode coupled to said secondsubstrate; at least one second electrode electrically coupled to saidfirst electrode; a first impedance coupled to said first electrode; asecond impedance coupled to said second electrode; a power supplycoupled to at least one of said electrodes; a first discharge eventdetector coupled to said first impedance; a second discharge eventdetector coupled to said second impedance; a plurality of pixels, eachpixel capable of outputting a gas discharge pulse upon interaction withradiation received from said photocathode, wherein each gas dischargepulse is counted as having an approximately equal value; and circuitryfor detecting if a gas discharge pulse is output from the pixels, andfor counting each gas discharge pulse as an individual event.
 2. Theradiation counting detector of claim 1, wherein said first electrode andsaid second electrode are electronically coupled to define each pixel.3. The radiation counting detector of claim 2, wherein said firstelectrode is an X-electrode and said second electrode is a Y-electrode.4. The radiation counting detector of claim 3, further comprising aprotective layer coupled to said photocathode layer.
 5. The radiationcounting detector of claim 3, further comprising a hermetic seal coupledto said first substrate and said second substrate, and peripheral edgespacers that define said gap.
 6. The radiation counting detector ofclaim 3, wherein said second electrode is coupled to said secondsubstrate.
 7. The radiation counting detector of claim 3, wherein saidsecond electrode is coupled to said first substrate.
 8. The radiationcounting detector of claim 6, wherein a gas discharge between said firstand second electrodes is a surface-discharge shape.
 9. The radiationcounting detector of claim 7, wherein a gas discharge between said firstand second substrates and at an intersection of said first and secondelectrodes is a columnar-discharge shape.
 10. A radiation countingdetector comprising: a first substrate; a second substrate generallyparallel to said first substrate and forming a gas gap with said firstsubstrate; a photocathode coupled to said first substrate and facingsaid second substrate; a dielectric layer coupled to the photocathode.at least one first electrode; at least one second electrode forming atleast one pixel with said first electrode; a first impedance coupled tosaid first electrode; a second impedance coupled to said secondelectrode; a power supply coupled to at least one of said electrodes;first circuitry coupled to said first impedance for detecting agas-discharge counting event in said first electrode; second circuitrycoupled to said second impedance for detecting the gas-dischargecounting event in said second electrode; and a plurality of pixels, eachpixel capable of outputting a gas discharge pulse upon interaction withradiation coming from said photocathode; wherein each gas dischargepulse is counted as having an approximately equal value; and wherein thefirst and second circuitry is adapted to detect if a gas discharge pulseis output from the pixels, and counting each gas discharge pulse as anindividual event.
 11. The radiation counting detector of claim 10,wherein said first substrate comprises a first and second side, furthercomprising a scintillation plate optically coupled to said first side,wherein said photocathode is coupled to said second side.
 12. Theradiation counting detector of claim 10, wherein said first substratecomprises a first and second side, further comprising a luminescentmaterial layer optically coupled to said first side, wherein saidphotocathode is coupled to said second side.
 13. The radiation countingdetector of claim 10, wherein said first electrode is coupled to saidfirst substrate and is a cathode, and said second electrode is coupledto said second plate and is an anode.
 14. The radiation countingdetector of claim 10, wherein said first electrode and said secondelectrode are both coupled to said second substrate.
 15. The radiationcounting detector of claim 10, further comprising at least onecurrent-limiting impedance coupled in series with each of said pixels.16. The radiation counting detector of claim 10, further comprising aninternal barrier wall structure to physically isolate said pixels. 17.The radiation counting detector of claim 14, wherein said power supplyis a direct current power supply.
 18. The radiation counting detector ofclaim 14, wherein said power supply is an alternating current powersupply.
 19. The radiation counting detector of claim 14, furthercomprising a closed cell internal microcavity pixel structure thatphysically isolates said pixels.
 20. A method of detecting radiationbased on a counting of gas discharge events comprising: receivingradiation at a first substrate of a plasma panel, said plasma panelhaving a second substrate; creating at least one free-electron within aphotocathode layer coupled to a side of said first substrate that facessaid second substrate, said creating in response to the receivedradiation and resulting in an emittance of an electron out of saidphotocathode layer and into a gas contained within a gap between saidfirst and second substrates; causing a gas-discharge event at a pixelsite of the plasma panel; and counting a plurality of said events at afirst gas-discharge pulse detector coupled to a first electrode and at asecond pulse detector coupled to a second electrode wherein each of saidevents is counted as approximately a same value.